When doing palatography, one needs something to paint the tongue with. It is usually a mixture of olive/sunflower oil and powdered charcoal. The question is, what can this substance be called? Is it a colouring, paint or/and dye? What is the most suitable word?
I would call it "a special non-toxic paint" (harmless if swallowed).
It was looking for what to call a mixture of oil plus pigment that gets "painted" onto the person's tongue and palate. Dye is permanent or semi-permanent as it penetrates fibers. Colo(u)rings change the color of a substance when mixed into the substance. Paint is a coating. So paint is an accurate name for it, and to make clear that it's not off-the-shelf house-paint, I called it "non-toxic".
A one word term for this is perhaps not available. However, there are several sources of which three are mentioned below that mention the two-word term "coloring agent" (BrE: colouring agent), but no other.
Wikipedia: It involves painting a coloring agent, such as a dye or a mixture of charcoal and olive oil on the tongue or the roof of a person's mouth and having that person pronounce a specific sound.
Wiktionary: A technique used to identify which parts of the mouth are used when making a certain sound, by painting a colouring agent on the tongue or roof of a subject's mouth and photographing those areas after the subject has pronounced the sound.
University of Sheffield: Palatography involves using a colouring agent (such as dye) on a speaker’s tongue or the roof of their mouth to identify which part of the mouth is used when producing different sounds. This method has been extensively used at UCLA.